1. Field of the Invention
This invention relates to in situ recovery of shale oil, and more particularly to a mining system for excavation and explosive expansion of oil shale formation in preparation for forming an in situ oil shale retort.
2. Description of the Prior Art
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; 4,043,598; and 4,192,554; which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted shale oil. One method of supplying hot retorting gases used for converting kerogen contained in oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
Techniques used for forming a fragmented mass can affect the uniformity of particle size or permeability of the fragmented mass. A fragmented mass having reasonably uniform permeability in horizontal planes across the fragmented mass can avoid bypassing portions of the fragmented mass by retorting gas, which can otherwise occur if there is gas channeling through the fragmented mass owing to non-uniform permeability. A fragmented mass having non-uniform permeability can be processed if techniques are provided for avoiding gas flow bypassing regions of low permeability in the fragmented mass.
In situ oil shale retorts have been devised with a drift communicating with the fragmented mass through a side boundary near the bottom of the retort. With such an arrangement, an appreciable quantity of oil shale in the fragmented mass can remain unretorted at the end of retorting operations. This problem can arise from several factors. Gas introduced into the fragmented mass tends to flow more or less directly toward the gas outlet. When the gas outlet is in the form of a drift opening through a side boundary of the retort, gas flow tends to concentrate toward that side of the retort, and an appreciable volume of oil shale adjacent the opposite side boundary near the bottom of the retort can be bypassed. Such oil shape bypassed by gas flow can remain unretorted. Further, in some techniques for forming the fragmented mass, some particles can flow or be driven into a drift communicating with the retort, resulting in a higher void fraction in a region above the drift than in other parts of the fragmented mass. Such a high void fraction can permit preferential gas flow or channelling and thereby bypass parts of the fragmented mass with lower void fraction.
Consequently, there is a need to develop techniques for avoiding yield losses in a retort having a lower level drift that communicates with the fragmented mass through a side boundary near the bottom of the retort. Such a retort can provide significant savings of mining costs since an economical two-level mining system can be used with an upper working level excavated adjacent to the upper boundary of the retort and a lower working level, or lower level drift at the lower boundary of the retort. Such a mining system can be more economical than a system having multiple intermediate level voids. For instance, U.S. Pat. No. 4,043,597 and 4,043,598 disclose methods for forming a fragmented mass in a horizontal free face system with intermediate level voids. The mining and construction costs involved in preparing a retort for explosive expansion can be reduced by eliminating excavation of multiple voids and corresponding retort level access drifts at intermediate levels of a retort site.
The present invention facilitates use of a two-level, horizontal free face mining system in which a fragmented mass can be formed without excavating multiple void spaces and corresponding retort level access drifts at different intermediate levels within a retort site. In addition, the present invention provides techniques for avoiding gas channeling in a fragmented mass having a non-uniform permeability distribution. The invention also provides techniques for significantly reducing gas channeling through a fragmented mass having a lower level drift communicating with a lower side boundary of the fragmented mass.